1. Field of the Invention
The invention relates to a rotary-anode X-ray tube comprising a slave bearing with a stationary bearing potion and a rotatable bearing portion, one of the facing surfaces of which is provided with a groove pattern, between the two bearing portions there being formed a bearing gap which is filled with a lubricant at least at the area of the groove pattern.
2. Description of the Related Art
In rotary-anode X-ray tubes of this kind a problem is encountered in that gas inclusions may be present in the lubricant, said gas inclusions being formed partly in the course of operation at high temperatures. The gas inclusions collect mainly at the area of low pressure, i.e. at the edges of the groove pattern. At that area they may join and displace the lubricant, thus affecting the operation of the sleeve bearing. Such gas inclusions become manifest up until the end of the evacuation process of the X-ray tube.
In order to eliminate such gas inclusions, in the X-ray tube which is known from EP-OS 552 808 a duct system is provided in the center of its bearing portion, which system is filled only partly with lubricant; the reminder of the duct system can accommodate the gas inclusions. In that case there is a risk of escape of lubricant from the duct system.
Furthermore, from EP-OS 479 198 a rotary-anode X-ray tube of the kind set forth is known in which the bearing gap continues outwards into a labyrinth in order to prevent the escape of lubricant. The labyrinth opens into a chamber which is sealed from the environment by a porous substance of a material which reacts with the lubricant. This porous substance can be traversed by gas inclusions whereas the lubricant is retained in the substance. When gas inclusions can reach the substance through the labyrinth, however, they can no longer affect the operation of the slave bearing any way. The effect on the gas inclusions at the area of the sleeve bearing, therefore, is limits. If the porous substance were used in the vicinity of the groove pattern, there would be a risk of not only the gas inclusions but also the lubricant reaching the porous substance in which it is retained and hence withdrawn from the sleeve bearing.